Optical projection printing is used in current semiconductor fabrication processes to transfer optical patterns from a transparent plate patterned with light-impermeable material ("a mask") onto a silicon substrate coated with a radiation sensitive medium, such as photoresist. During exposure the resist coated substrate is located at the image plane of an imaging system, such as an optical lithography system. Integrated circuits are built up in a series of conductive and insulative layers, each layer being patterned through photoresist exposure and subsequent etching through the patterned photoresist.
For proper functionality of the circuits the size of the photoresist image must be tightly controlled because photoresist image size determines the sizes of structures, such as devices, gates, contacts, conductors, and insulators comprising the circuit. The photoresist image size is mostly a function of exposure dose and photoresist contrast. Since most photoresists have high contrast, the imaging process can be approximated by a threshold model: below a certain threshold dose of light, none of the exposed photoresist is activated, above the same threshold, all of the exposed photoresist is activated. Thus, upon developing positive-tone photoresist, all the resist exposed with a dose above the threshold is removed, while all the resist exposed with a dose below the threshold remains.
In addition to exposure and contrast, the masking pattern itself can influence photoresist image size. This is especially the case if images within the masking pattern (mask images) are near the resolution limit of the system's imaging capability. In this regime, the exposure threshold depends on specific features of the masking pattern. For example, the exposure dose required to reproduce (or "print") a masking pattern consisting of a single opaque line at or near the resolution limit is higher than that required to print a masking pattern consisting of a group of equi-spaced opaque lines of the same size. The exposure dose required for a masking pattern consisting of a single transparent line of the same size is still different. The difference in exposure threshold between different masking patterns all having mask images that are the same size leads to size differences between the photoresist images when they are all printed with the same exposure dose. Therefore, a mask containing a variety of patterns with mask images at or near the resolution limit of the lens will not be uniformly reproduced in the photoresist by a single exposure.
There has been considerable effort devoted to eliminating this masking pattern-dependent variation in exposure threshold. Commonly assigned U.S. Pat. No. 4,456,371 by Lin (the "'371 patent") teaches several methods of exposing the wafer with a correction mask tailored to the standard mask so that from both the desired exposure is achieved. Each approach requires fabrication of two masks and careful alignment of the two masks.
In an article "Binary and Phase-Shifting Image Design for Optical Lithography", Proceeding of the SPIE, Optical/Laser Microlithography IV, Vol. 1463, (1991) pp. 382-399, Liu and Zakhor teach predistorting images on the mask. The mask is generated using a computer simulation of an optical lithography system that includes the type of mask illumination, the exposure dose, the imaging lens, and the desired photoresist pattern.
U.S. Pat. No. 4,869,999 (the "'999 patent") to Fukuda et al. teaches a method of image enhancement by performing multiple exposures at different focus settings. The effective depth-of-focus of an optical lithography system is found to be improved by repeating exposures focused at various depths within the depth of focus of the imaging system.
A simplified way of minimizing masking pattern-dependent variations in substrate image size that occur in printing mask images that are at or near the resolution limit of the optical lithography system is needed and is provided by the present invention.